OPTIMISTIC GROWTH: COMPETITION AND AN ONTOGENETIC NICHE-SHIFT SELECT FOR RAPID GROWTH IN PUMPKINSEED SUNFISH (LEPOMIS GIBBOSUS)

Intrinsic growth rate is emerging as an important life-history trait that can be modified by natural selection. One factor determining optimal intrinsic growth rates is the pattern of resource availability. Organisms that experience chronically low resource levels tend to have slow intrinsic growth rates. However, this does not necessarily hold if resource levels change as an organism grows. We present a theoretical model showing that rapid growth is favored when resource levels for small size classes are low relative to resource levels for large size classes. We call such a growth strategy “optimistic” because rapid growth is based on an expectation that resources will improve once a minimum size is reached. We provide empirical support for this hypothesis by examining the intrinsic growth rates of pumpkinseed sunfish derived from three populations sympatric with bluegill sunfish (an important competitor with small size classes) to three populations allopatric with bluegill sunfish raised under common conditions. Rapid growth has evolved in the sympatric fish to reach the size refuge from competition as quickly as possible.